How can I make both sides of this mesh bright in material mode?
Every face has a normal- a direction that shows where "outside" is. By this I mean that when multiple faces are joined into an object, it tells blender that the rest of the space is outside and inside the object is inside. Blender shows the inside portion darker so you know  where your normals are facing. Unreal engine will make backside normals invisible, so I would suggest a solidify modifier with a very thin thickness. The wing will remain very thin, but not have any normal issues.